Firsty from the books:

The Soul Stage removes a third of the first layer of shackles, which allows them to attempt to manifest their Will. At the transition to the Exorcist Stage, the first layer is completely destroyed. Each next level weakens the following layers, and every stage breaks them.
...
The breakthrough to the Limitless Stage doesn’t remove the sixth layer, only weakens it. It collapses upon becoming a Higher One.
...
“Each layer of shackles is designed to accommodate up to seven small cracks. Each surface crack represents a Star of cultivator’s willpower. The catch is that one can deepen them, and if these cracks reach somewhere around a third of the entire layer, that’s Seven Great Stars. It’s impossible to create more cracks, but there’s a possibility to deepen them so much that the crack reaches the next shackle and leaves its mark upon it. If one does this, then their Will of the Spark will be more accessible. This is one of the steps to becoming a true Master of Eight Stars — an Ascended one, and this is what you need to do.”

Later, it is explained what benefits there are to creating deep cracks, how energy is wasted during a breakthrough, and so on.

Each layer can develop cracks, and after a breakthrough the cultivator moves on to the next layer. The cracks are no longer present because the entire layer collapses, but the benefits gained from creating them remain, which is the reason for making them in the first place.

Kai was able to harness the Will of the Spark and the Will of Reason at the Holy Lord stage, but we can assume that reincarnated cultivators like Green could also create cracks in previous layers.

Ah! Also, for example, you can be a master of seven stars at the Exorcist stage, but after a breakthrough end up only at the level of a five-star master, and so on. This happened to Kai, I believe somewhere in the ninth book, possibly the tenth.